It is often important to determine the amount of nitrogen in an object containing nitrogenous compounds and non-nitrogenous compounds. Nitrogen is a common element of many important compounds. For example, nitrogen is a key element in proteins, certain plastics, fertilizers, explosives, certain medications, and even illegal drugs. The art has proposed detection of nitrogenous compounds by gamma ray scattering or absorption by the nitrogen of the compounds, and that scattering or absorption is detected and analyzed for determining the amount of nitrogen in an object of interest. Very briefly, in those techniques, the object of interest having nitrogenous compounds therein is placed in the path of a gamma ray beam. When the beam is of required characteristics, the gamma rays are resonantly absorbed or scattered by the nitrogen in the object being examined, and those gamma rays are detected by conventional detectors which provide output signals proportional to the amount of nitrogen in the object being examined. The details of the theory of such detection devices are well known, and a rather complete description of the theory is presented in U.S. Pat. Nos. 4,941,162 and 5,040,200, which patents are incorporated herein by reference and relied upon for the details of the theory and conventional apparatus for carrying out gamma ray detection of nitrogenous compounds.
However, there have been common difficulties in this prior art in that gamma ray scattering or absorption are subject to special limitations in connection with the manner in which the gamma ray beam is applied and in connection with suitable detectors for the transmitted gamma rays. A representative example of those difficulties is disclosed in U.S. Pat. No. 4,941,162. That patent is, specifically, directed to the detection of nitrogenous explosive material, e.g. explosive material concealed in a travel bag, e.g. luggage. In the arrangement of that patent, a gamma ray source emits gamma rays of a desired and monitored flux, and those gamma rays are limited in divergence by a collimator so that a beam of gamma rays will intersect luggage moving on a conveyor near the collimator. Explosive material (which will normally contain nitrogenous compounds) will cause gamma ray attenuation, indicating the presence of nitrogenous compounds in the luggage.
However, that arrangement has two very decided disadvantages. Firstly, in that arrangement, the attenuation of the gamma rays by nuclear resonance takes place with non-resonant attenuation. Accordingly, in order to determine the presence of or amount of nitrogenous compound, that arrangement requires two different sets of detectors, i.e. one set of detectors which can determine the resonant attenuation and one set of detectors which can determine the non-resonant attenuation. These two sets of detectors are linked to a data analysis system which then uses both the data from the resonant detectors and the non-resonant detectors to make a determination of the presence of nitrogenous compounds. This considerably complicates the detection of nitrogenous compounds and, as a result, considerably lessens the accuracy of that determination.
A further difficulty of that arrangement, which is common to the art, is that a result of that arrangement is the necessity to use nitrogen-rich detectors, which, according to the prior art, were required. This was thought to be required because nuclear resonant scattering requires resonant detectors to select the relevant energy portion of the transmitted flux spectrum which contains the resonance absorption information, and it was thought that nitrogen-rich detectors were required to achieve that function.
Further, and partly as a result of the foregoing, the accuracy of detection of nitrogenous compounds is not to the degree required, and to ensure that luggage does not contain nitrogenous explosives, it is necessary in that prior art arrangement to orient the luggage, with respect to the gamma ray beam, at various angles, as the luggage passes through that beam. This considerably complicates and slows the examination of luggage. For example, if the nitrogenous explosive material is in thin sheet form, and the luggage passes that beam with the thin sheet perpendicular to the beam, the lack of desired accuracy could easily not detect the presence of the nitrogenous explosive material in thin sheet form. Thus, it is necessary in that prior art arrangement to examine the luggage as it is passed through the beam a plurality of times, but at different angles to the beam.
While this lack of desired degree of accuracy of detection could be quite dangerous in regard to luggage to be boarded on an aircraft, that undesired degree of detection is also serious in other detections of nitrogenous compounds. For example, the same degree of detection provided by that arrangement could also fail to detect illegal drugs in a particular design of a container for the drugs. Somewhat similarly, for example, where the protein content of a flowing dairy product, such as milk, is to be detected, that degree of detection could give erroneous protein content results. Also similarly, where the protein content of a grain or cereal is to be detected, that degree of accuracy could also give erroneous results.
As can, therefore, be appreciated, it would be of substantial advantage in the art to provide apparatus and methods for gamma ray detection of nitrogenous compounds in an object also having non-nitrogenous compounds, where the accuracy of the detection is considerably increased. It would also be of substantial advantage to the art to provide such apparatus and methods where the apparatus and method are less complex and do not require separate determinations of resonant attenuation and non-resonant attenuation, with the attendant necessity of separate arrays of resonant detectors and non-resonant detectors.